The present invention relates to a matrix calculating circuit capable of removing, at a high speed, the adverse effect by an interference among a plurality of data items, if the interference occurs, and to an image sensor using the matrix calculating circuit. More particularly, the invention relates to a matrix calculating circuit capable of calculating, in real time, such a simple matrix relation that in calculating a value from one data item of a data stream consisting of "n" number of data items that are time-sequentially outputted, when the value is influenced by another data item in the data stream, the characteristics of the systems interfering with one another are expressed by the matrix relation, and all the diagonal elements of the matrix relation are equal to one another, and all the remaining elements are also equal to one another.
An example of the case where a plurality of data items interfere with one another is an image sensor of the type in which a number of photodetecting elements are grouped into a plurality of blocks, and those blocks are matrix-driven for each block by switching elements.
This type of the image sensor includes "K" number of linear arrays of blocks consisting of photodetecting elements 3 each being equivalently expressed by the combination of a photo diode 1 and a capacitor 2, as shown in FIG. 3. Light reflects from the image bearing surface of a document according to a density D of an image on the document, hits the photo diodes and causes them to produce photo currents. By the photo currents, image information on the document surface is stored in the capacitors 2 in the form of charge quantity Q. To read the image information, switching elements 4 are closed every block, so that the charge Q is transferred to wiring capacitors 7 of "n" number of common wires 6 coupled with an analog multiplexer 5. Then, the signals derived from a shift register 8 sequentially close switching elements 9. A voltage V varying depending on the charge Q that has been transferred to and stored in the wiring capacitors 7 is picked up time-sequentially. The picked-up signal is passed through an A/D converter 10 where the signal is converted into a digital value of 8 bits, for example, and then the digital value is outputted, as digital image data, onto a signal line 11.
Thus, to read the image information, the density D as the image data is first converted into the charge Q proportional to the density D, and the charge Q is further converted into the voltage V proportional to the charge Q.
The image sensor employs a matrix wiring structure 13. In the wiring structure, the upper and lower wires are arrayed in a matrix, and an insulating layer is interposed therebetween. With such a structure, lead wires 12 led from the photodetecting elements 3 are connected to common wires 6 for each block. Because of the structure, coupling capacitance is caused at each of the cross points of the wires. During the charge transfer, the coupling capacitance seizes the charge, so that the entire charge (image information) stored in the capacitor 2 cannot be transferred to the wiring capacitor 7.
An equivalent circuit of the wire portion of the image sensor, when the coupling capacitances 14 are taken into consideration, may be expressed as shown in FIG. 4. In the circuit, the lead wires 12 and the common wires 6 are laid out so that the coupling capacitances 14 are equal to one another in value.
It is assumed that no coupling capacitance is present between the signal lines consisting of the lead wires 12 and the common wires 6. On this assumption, charge Qi, which is stored in the capacitor 2 of capacitance CP by the photo diode of the i-th photodetecting element of the "n" number of photodetecting elements, is given by the following equation. EQU Qi=CPVi.phi.
It is assumed that the capacitance of the wiring capacitor 7 is CL when no crosstalk is present. Then, if the switching element 4 is closed, the charge Qi is expressed by the following equation. EQU Qi=CPVi.phi.=(CP+CL)Vi
where Vi is the output voltage.
The above equation showing the proportional relationship of the charge Qi and the voltage Vi teaches that by reading the voltage Vi, one can ascertain the density D, which is proportional to the charge Q. However, the crosstalk is actually present, and the charge Qi is not proportional to the voltage Vi.
In an actual image sensor, the charge Qi is stored being distributed into the capacitance CP of the i-th capacitor 2, the wiring capacitance CL, and the coupling capacitance Cc present between the wires and the (n-1) number of common signal lines 6. The charge Qi and the output voltage Vi (i=1 to n) satisfy the following matrix relation. ##EQU1##
Accordingly, in order to exactly read the density D, the charge Qi containing the crosstalk component must be calculated from the voltage Vi by using the above matrix relation.
To realize this, a circuit as shown in FIG. 5 has been used. In FIG. 5, the output value Vi (image data) of the A/D converter 10 is outputted through a port 15 to a data bus 16. A CPU 17 fetches the data and stores it to a memory 18. Then, the CPU performs the calculation of the equation (1) under control of a software for the calculation.
However, the calculation of the equation (1) by the CPU 17 under the software control takes much time, hindering high speed read of image data. Use of the CPU 17 and memory 18 increases the device size and hence cost to manufacture the device.